Minor windows cleanup

Hello,

This pull request includes several fixes for outstanding issues found by users using Heroku Toolbelt in Windows:

- Completely isolate Ruby installation bundled from any other installation in the user computer. Fixes #14
- Made both `heroku.bat` and `heroku` (bash) smart enough to use bundled Ruby: https://gist.github.com/4668599
- Do not set environment variable `HOME` since it should be automatically discovered by `git.exe` (this needs newer version of msysGit). Fixes #28
- Do not add posix-compatible tools from Git that clash with Windows one

However, this pull request do not solve:

- Upgrade of Ruby 1.9.2: I recommend upgrade to Ruby 1.9.3-p327 at minimum
- Upgrade of msysGit: I recommend upgrade to 1.8.0 which works perfectly with both ssh and https authentication

Please consider merge before a new release.

Thank you
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